At some places there is a demand for controlling the flow of vehicles, and one way to perform such a control is to use fee collection systems, where vehicles passing a road side station are debited a certain fee, where the fee may vary, by way of example for different types of vehicles and for different times.
Normally, each road side station is equipped to communicate to vehicle transponders, which are positioned in the vehicles. Each transponder is normally arranged to receive a signal from the toll station and to respond, where the response for example comprises vehicle identification data.
Each transponder normally comprises a switch that is arranged to switch between transmission and reception of the active radio unit and, if the transponder comprises a single antenna, for example a switch or directional coupler to couple the antenna to either the radio unit or wake-up circuitry depending on the mode of operation.
A battery powered transponder is suitably equipped with power saving means that is arranged to start a power saving mode, where a certain stimuli signal wakes the transponder when it is in power saving mode. In this way, the transponder may save battery energy. In the power saving mode, the antenna is coupled to the wake-up circuitry.
Furthermore, in order to control transmitted output power from an active radio unit, a transponder is normally equipped with a directional coupler on the output, where a predetermined part of the transmitted power is fed back to a radio unit in the transponder, allowing the radio unit to control the transmitted output power.
However, a transponder with power feedback and a wake-up circuitry comprises a number of parts as indicated above, and there is a desire to obtain a less complex arrangement for obtaining the same functionality in a transponder.